The present invention relates to a method of and device for controlling the welding process in the manufacture of pipes with longitudinal seams by high-frequency welding a seam along a skelp that is continuously shaped into an open-seamed pipe, with the amount of electric power supplied, the welding pressure that forces the edges of the skelp together, and the welding temperature being maintained within ranges at which the seams will weld together properly.
The invention is based on that described in U.S. Pat. No. 3 573 416, from which measuring and controlling the welding pressure and the amount of electric power supplied is known.
The disadvantage to that method is that welding pressure can not be controlled in accordance with its result: the mass of the welding bead.
German Auslegeschrift No. 2 850 978 discloses a method of controlling welding pressure in accordance with bead mass by non-contact measurement of the width of the bead directly behind the welding point with a diode-matrix camera. This method of course assumes that the mass is proportional to the width. The same publication also discloses a variation of a known circuit for controlling the output of high-frequency welding equipment by combining measurements of temperature and bead width.
The method is, however, expensive and imprecise. Neither the amount of welding energy that must be supplied moment by moment to obtain a welding process that is as linear as possible nor the level of welding pressure can be satisfactorily determined from the width of the welding bead.
It is known from German Offenlegungsschrift No. 2 826 986 that, because the welding point does not advance at a constant rate, the bead that results from the high-frequency welding of longitudinal pipe seams etc. looks when viewed from above like a densely packed series of non-spherical globules of constantly increasing size with flattened contact surfaces. Thus, the mass of the bead differs continously.